G-protein coupled receptors are cell surface receptors that indirectly transduce extracellular signals to downstream effectors, e.g., intracellular signaling proteins, enzymes, or channels. Changes in the activity of these effectors then mediate subsequent cellular events. The interaction between the receptor and the downstream effector is mediated by a G-protein, a heterotrimeric protein that binds GTP. Examples of mammalian G proteins include Gi, Go, Gq, Gs, and Gt (for a review, see, e.g., Morris and Malbon, Physiol. Reviews 79: 1373–1430; 1999).
G-protein coupled receptors (“GPCRs”) typically have seven transmembrane regions, along with an extracellular domain and a cytoplasmic tail at the C-terminus. These receptors form a large superfamily of related receptor molecules that play a key role in many signaling processes, such as sensory and hormonal signal transduction. The further identification of GPCRs and the natural ligands of the receptors is important for understanding the normal process of signal transduction as well as theirs involvement in pathologic processes. For example, GPCRs can be used for disease diagnosis as well as for drug discovery. GPCR ligands may be used for the treatment of GPCR-related disorders and for the identification of additional modulators of GPCR activity. Further identification of GPCRs and ligands that bind to GPCRs is therefore of great interest.
Nicotinic acid (vitamin B3, also known as Niacin) is an essential dietary ingredient, its deficiency causes pellagra in humans. It is incorporated into either of two coenzymes, nicotinamide adenine dinucleotide (NAD) or nicotinamide adenine dinucleotide phosphate (NADP). Many biochemical reactions in glucose, lipid and protein metabolism are dependent on these coenzymes.
Nicotinic acid, at gram dose, has also been used as a drug for the treatment of hyperlipidemia, e.g., acipimox. It is effective in reducing total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), triglycerides (TG) and lipoprotein (a) [Lp(a)]. At the same time, it also increases high-density lipoprotein cholesterol (HDL-C). Because epidemiological studies have established that the risk of coronary heart disease is linked positively to the levels of TC and LDL-C, and inversely to the level of HDL-C, the effects of nicotinic acid are highly desirable. Recent clinical studies also indicate that a combination of nicotinic acid therapy with statins has added benefit.
The mechanism by which nicotinic acid alters lipid profiles has not been well defined. It may involve several actions including inhibition of lipolysis in adipocytes, inhibition of release of free fatty acids from adipose tissue, and increase of lipoprotein lipase activity. The action of nicotinic acid on adipocytes may be mediated by inhibition of adenylyl cyclase through pertussis toxin-sensitive G proteins. A binding site for nicotinic acid in adipose tissues was recently described, but not yet characterized on molecular level. The most prominent side effects of nicotinic acid are skin flushing and itching on the face and neck. This “niacin flush” can be very uncomfortable, and greatly limits its use. Although not fully proved, it is proposed that the side effect of skin flush is mediated by vasodilating prostaglandin D2 released from macrophages surrounding cutaneous blood vessels. Supporting this proposal, a binding site for nicotinic acid has been detected in mouse macrophages.
Nicotinic acid derivatives are also used as vasodilators to treat peripheral vascular disease and other disorders of circulation, including Raynaud's syndrome. They can reduce concentrations of fibrinogen and lower blood viscosity (the fluidity of the blood).
Without knowing its molecular target(s), it is almost impossible to improve the potency, efficacy and specificity of nicotinic acid in order to obtain a superior medicine with better therapeutic and side effect profiles. The current invention is based on the discovery that nicotinic acid is a ligand for two GPCRs.